This invention relates to liquid ring pumps, and more particularly to improved intrastage and interstage seals for two-stage liquid ring pumps.
Two-stage liquid ring pumps are shown for example in Fitch U.S. Pat. No. 4,132,504, entitled "Liquid Ring Pump." Pumps of this kind typically include axially adjacent first and second stages. Each stage includes a cylindrical housing having a port plate on at least one end of the housing. Each port plate has separate gas intake and discharge ports. (It will be understood that the term "gas" as used herein includes vapor. Also, although various arrangements of intake and discharge ports may be used, it will be assumed for convenience herein that each port plate has only one intake port and one discharge port.) In the pumps of particular interest to the present invention, each stage includes such a port plate on at least the end of the housing adjacent the other stage. Pumps of this type are known as center head pumps, and the two adjacent port plates (one in each of the two stages of the pump) are sometimes referred to for convenience herein as center port plates. A rotor is eccentrically mounted for rotation in the housing for each stage. Each rotor includes a plurality of blades extending radially outward from a hub. The hubs of both rotors are mounted on a common shaft which extends through the center port plates. A quantity of pumping liquid (e.g., water) is also present in each stage.
In operation, the common shaft is rotated to rotate the rotors and thereby produce a ring of circulating pumping liquid in each stage. Because each rotor is mounted eccentrically in its housing, the rotor blades cooperate with the pumping liquid in each stage to form pumping chambers between adjacent blades which cyclically expand and contract as the rotor rotates. The intake port of each stage communicates with the expanding pumping chambers, while the discharge port communicates with the contracting chambers. Accordingly, gas is drawn into each stage via the intake port at a relatively low pressure and is discharged from that stage via the discharge port at a higher pressure. The discharge port of the first stage is connected to the intake port of the second stage so that the first stage discharge pressure is further increased by operation of the second stage, i.e., the second stage discharge pressure is higher than the first stage discharge pressure.
In liquid ring pumps it is generally desirable to increase the volumetric efficiency of the pump by reducing the leakage of gas from the higher pressure zones in the pump to the lower pressure zones. In two-stage pumps of the center head type this leakage is typically of two kinds: intrastage leakage from the higher pressure side of each stage to the lower pressure side of that stage, and interstage leakage from the second stage to the first stage. Intrastage leakage chiefly occurs at the end of the rotor hub adjacent a port plate. Interstage leakage chiefly occurs along the shaft extending between the intrastage leakage sites described above.
Both intrastage and interstage leakage can be reduced by building the pump with small clearances between the hub ends and the adjacent port plates and with small clearances for the interstage portion of the shaft. However, such small clearances substantially increase the manufacturing cost of the pump and are unlikely to be completely effective in eliminating leakage.
To reduce intrastage leakage it is known to introduce a flow of sealing liquid (generally the same as the pumping liquid) into the clearance between the port plate and the adjacent hub end. This, however, may have several disadvantages and shortcomings. First, a relatively large flow of such sealing liquid may be required to attempt to substantially fill the clearance. Second, the sealing liquid has a strong tendency to flow only toward the low pressure side of the pump which may allow intermittent failure of the seal. Third, and especially important in the first stage of two-stage pumps pumping gas which is initially less than fully saturated with vapor of the sealing liquid, the strong flow of sealing liquid toward the low pressure side of the pump typically introduces a spray of sealing liquid into the gas as it enters the pump via the intake port. This is highly conducive to saturating the intake gas with sealing liquid vapor before the gas has been significantly compressed by the pump, thereby reducing the volumetric efficiency of the pump.
Many of these shortcomings of intrastage liquid seals are aggravated in two-stage pumps of the center-head type by the possibility of interstage leakage.
In view of the foregoing, it is an object of this invention to improve and simplify two-stage liquid ring pumps of the center head type.
It is a more particular object of this invention to increase volumetric efficiency and reduce both intrastage and interstage gas leakage in two-stage liquid ring pumps of the center head type.